Problems with link libraries for ecl_time tests

[bit-pirate]

E.g.

-- skipping gtest 'test_sleep' in project 'ecl_time'
CMake Error at ecl_core/ecl_time/src/test/CMakeLists.txt:9 (target_link_libraries):
  Cannot specify link libraries for target "test_sleep" which is not built by
  this project.


-- Configuring incomplete, errors occurred!```

[bit-pirate]

We just found out that this problem only occurs, if gtest is not installed. In this case the target is not build (by catkin) and hence linking it fails.

One solution is adding:

catkin_add_gtest(test_cpuwatch cpuwatch_rt.cpp)
if(TARGET test_cpuwatch)
  target_link_libraries(test_cpuwatch ${PROJECT_NAME})
endif()

[stonier]

Solution works: there are lots of commits everywhere to bring these up to speed and probably a few more to go, but we seem to be mostly covered now.